1. Technical Field
The present invention pertains to the field of industrial control automation. More particularly, the present invention pertains to communication over a network between a programmable logic controller (PLC) of an industrial control system and network I/O devices connected to control and monitoring elements of the industrial control system, as well as communication between different PLCs of a distributed industrial control system.
2. Description of Related Art
As indicated in FIG. 1, an industrial control system often includes a programmable logic controller (PLC) for providing coordinated control of industrial control equipment, which includes various elements, that are often either sensors for providing inputs to the PLC or relays for receiving outputs from the PLC, each under the control of an element controller, and each connected to the network via a network I/O device. Industrial control using a PLC requires what is termed rapid scanning, meaning the continuous, rapid execution by the PLC of three main steps executed repeatedly: the acquiring of the status of each input to the PLC needed to execute so-called ladder logic for the process being controlled, the solving of the ladder logic to determine each output, and the updating of the status of the outputs.
The term ladder logic is used to indicate, in a form recognizable to early workers in the field of machine control, the expression of how the control elements of an industrial control system are to be controlled based on the monitoring elements of the industrial control system. The term ladder is used because the expression of the control logic is actually often in the form of a ladder, with each rung of the ladder having an output, i.e. a value for the required state of a control element, and one or more inputs, i.e. values corresponding to signals from monitoring elements. The inputs for an output may be in series, in which case they span a rung of the ladder, or in parallel, in which case the term ladder is not so apt.
For effective industrial control, a PLC must communicate with the monitoring elements of the industrial control system, execute the ladder logic, and provide the outputs to the control elements at a rate that is adequate for effective industrial control. In this scanning (input, solve, output), usually only a small amount of information is passed between a PLC and an element (via the network I/O device for the element and the controller for the element), but communication between the PLC and the element is usually frequent, once each scan. Off-the-shelf networks, such as Ethernet-type networks, have so far been found inadequate; setting up a communication, as would normally be done using such a network, takes too long for industrial control.
But from a cost standpoint, a general purpose off-the-shelf network is almost always preferable compared to a network designed specifically for a particular industrial control application, or even for a network designed for a generic industrial control application. Moreover, a mature general purpose off-the-shelf network should be more reliable than a custom network. Finally, using a general-purpose off-the-shelf network for an industrial control system allows also connecting at the same time computing hardware and software not related to the industrial control system. Using an off-the-shelf general purpose network, such as Ethernet, makes possible a single network strategy for unified communications from device to site level. With Ethernet, even worldwide communications, at least for monitoring, are possible via connection to the Internet.
What is needed is a way to operate a PLC over a general purpose off-the-shelf computer so as to allow scanning, i.e. execution of the three basic steps of a PLC, at a rate adequate for effective industrial control.